Likely more than half of the saltwater fish species that we keep are planktivorous in the wild. They rely on currents and hapless invertebrates in the water column for sustenance. The most popular fish groups, including the fancy basses (Anthiinae), fusiliers (Caesionidae), most of the damselfishes (Pomacentridae) and butterflyfishes (Chaetodontidae), are principally planktivorous.
While it is certainly known that almost all planktivores can and will learn to accept other fare in captivity, a good deal is lost in not providing plankton on a regular basis. Color, growth, behavioral activity and probably reproductive fitness all suffer from a lack of plankton. Providing this fodder is not difficult. Aquarists can easily feed these organisms (live, dried, frozen, pellets, sticks, etc.) to planktivorous fish a few times daily during “light hours” when these fish actively seek such foods. Store-bought or do-it-yourself plankton feeders can meter out dried planktonic foods or fluid for you. These systems can be simple in-tank or out-of-tank reservoirs that pump or drip into your display tank with a mix of foods suspended in solution, and some of these can be complex metering systems with timers.
Classically, plankton has been defined as an animal or plant (typically very small) in the water column that passively floats or feebly swims in the water. Investigating this huge mass of material reveals it to be made up of the following:
- Microscopic viruses and viroids
- Monerans (bacteria, fungi, blue-green algae)
- Protists of all sorts (including algae and protozoans)
- Metazoans (animals), with almost all phyla represented
- Sex cells (gametes) and many, many tons of their intermediate developmental life-stage individuals
Think of how many species are dependent on this floating biomass for sustenance. The three largest fish (whale shark, basking shark and manta ray), the largest whales (baleens), myriad sponges, sea squirts and most corals derive the bulk of their nutrition from planktonic sources. And nearly all larval marine fish are planktivorous as well as planktonic.
Phytoplankton is plant-based plankton. Animal types (mostly crustaceans, worms and mollusks) are zooplankton.
Plankton is also classified according to size, and this can be extremely important, as their potential predators often can only catch a given size range. They are further delineated by various aspects of their biology (e.g., bioluminescence).
This means that there are various types and formats of planktonic foods that can be manually or automatically added to your system. Some of the foods available are highly palatable and nutritious. All of these varieties and means of delivery have potential downsides and limitations. Feeders can and do fail at times, and the foods run out. Luckily, there is one great alternative: the use of refugiums.
Using Refugia to Produce Food
There are several important uses for refugiums, and one is that they can produce both phyto- and zooplankton. I want to be clear regarding phytoplankton and zooplankton; they can be easily produced in sufficient quantities for almost all sizes and types of reasonably stocked systems. In particular, in regards to phytoplankton, there is no need to supplement with other plankton sources, unless your system is overstocked with life, which needs more food than the refugium can produce.
Guidelines for Optimizing Your Refugium Production:
Size.Your refugium should be as large as possible. There can’t be too big a “fuge,” but the opposite is not so. Twenty percent of the display system is a good minimum for space dedicated for culture.
Substrate. Several inches of carbonate-based sand, calcium carbonate-based live rock (or base) and macroalgae (often Chaetomorpha, Gracilaria spp.) should be stocked. About once a year, replenish the sand and live rock as they “melt down” (dissolve and go into solution). Having all these media present will optimize your planktonic production by providing the chemical constituents that are incorporated into the bodies of the plankton. The media also help stabilize many aspects of water quality.
Skimmers. Efficient skimming can remove a good deal of smaller plankton, particularly phytoplankton. If you have a big skimmer, consider running it on a punctuated basis (a few hours on, a few hours off) or do what you can to discharge water from the refugium distally to the pickup of the skimmer itself.
Flow rate. Flow rates should be slow. Three to five times turnover per hour (of the refugium itself) is about right. This gives enough circulation to allow for gas exchange.
Lighting. Provide low illumination (fluorescents of any kind are fine) on a reverse photoperiod with your main display system; this means that the refugium lights are off when the display tank’s are on and vice versa. This is important not only for growing photosynthetic life but also for timing the release of much of the life to be swept into the display tank. Almost all plankton biotic activity occurs at night, and when they are active, they are out and are able to be sucked up by the pumps and put into the main display. Because your fish are active during the day, the refugium lights need to be off during the day so that the plankton move toward the display tank.
Stocking. In most cases, you won’t need to add any life to your refugium. The “critters” that come in on your live rock will populate the rest of the refugium. Just the same, some folks prefer to buy a “fuge livestock kit” from commercial sources, or borrow a scoop or two of sand from a fellow reefer to start off their biota. Unless there is some pressing reason, other types of life (predators) should be left out of your refugium. Crabs, large shrimp, snails, sea stars and fish of any kind will detract from the refugium’s production and utility.
Feeding. Most aquarists eschew purposely putting foods into their refugia. They instead count on spill-over from the main system and photosynthesis for plankton to feed on. But I encourage you to introduce at least a bit of dried food (sinking pellets are my preference) on a daily basis.
The use of live planktonic foods is of tremendous benefit in providing fish with nutrition and exercise, and it promotes overall health to captive marine systems. Among other means, the ongoing production and delivery of such foods through refugia is the best method available; it is consistently dependable, of moderate cost, and it provides a few other substantive advantages, such as keeping harassed tankmates, a location for placing a deep sandbed with all its itinerant uses (denitrification, alkaline earth replacement, buffering capacity) and an area for increasing gaseous diffusion. All display systems can be improved via the use of refugiums, which incorporate deep sandbeds, live rock, macroalgae culture and a reverse daylight photoperiod.
Bob Fenner is a well-known hobbyist and the author of the popular book The Conscientious Marine Aquarist. He has spent most of his adult life in the aquarium hobby, and owns and manages wetwebmedia.com.